Trump is talking about incarcerating and detaining all the undocumented immigrants in the United States, which would be about 11 million people. Snopes does a good job of fact checking the specific things he has said about this.
A November 2023 New York Times investigation found that Trump was planning “an extreme expansion” of his first-term immigration policies, including rounding up undocumented people in the United States and putting them in detention camps before expelling them from the country…
Trump had made similar comments in September 2023, saying he would carry out “the largest domestic deportation operation in American history.” He also said he would be, “Following the Eisenhower Model” for such deportations, referencing a 1954 campaign to round up and expel Mexican migrants named for an ethnic slur—Operation Wetback.
In sum, in December 2023 speeches, Trump did call for mass deportations and emphasized that such migrants come from all over the world, using rhetoric that echoed past speeches. Furthermore, reporting confirms that there are indeed plans for a future Trump administration to build huge detention camps to hold migrants.
Snopes
It is not hearsay that Trump is talking about these things – he gave an interview to Time (covered here, confusingly, by CNN) in which he talked about this explicitly. He says he plans to round 15-20 million people up into detention camps and deport them.
Some people say the Trump-Hitler comparisons are overblown, but it is worth remembering that Hitler had schemes to deport Jews to Madagascar, Alaska, and/or Siberia among other places, before he gave up on those and came up with his “final solution”. The other parallel is that he managed to bring local police and security forces under his central control, which echoes Trump’s discussion of using the National Guard and military here.
The human rights abuses this would engender are fairly obvious, but there are also many practical issues. Citizens of the United States are not required to carry papers proving their citizenship, so who do you choose to detain and how do you go about proving that they are undocumented? It would likely be done on the basis of (perceived) ethnicity, which brings to mind the incarceration of Japanese-Americans during World War II and the mistaken deportation of Mexican Americans in the 1950s. You could swoop in and interrogate inmates of jails and prisons, which is probably what would actually happen, but this would not add up to millions of people. The other thing you can do, and the U.S. government has done, is swoop into work places and demand to see papers. This is actually somewhat practical since workers are in fact required to prove they are eligible to work. However, being ineligible to work does not automatically mean your presence in the country is illegal. So now people might be swept up and held in detention camps without due process while courts try to figure out who they are. Even if a legal proceeding determines that an individual is in the country illegally, there is the problem of where to deport that person to (once again, see Hitler). Again, people could be held for years without due process while this plays out.
Finally, if you want to make inflation worse, deporting a huge chunk of the low-wage work force is a great way to do that. It would be much more humane and pro-business to expand work permits and temporary visas to let people in who want to work, but put some restrictions on how long they can stay and who they can bring along. This would be a win for human rights and would be business friendly, which at least traditional Republicans are nominally in favor of. The other side of this coin is to encourage U.S. business investment in Latin America, which would have the twin benefits of reducing migration pressure and producing cheap stuff to import, which also helps to hold down prices.